AS Booklet 4- Cell Recognition and the Immune System

Question 1

What is a pathogen?

Answer 1

A disease-causing microorganism.

Question 2

What are the defence mechanisms of the human body?

Answer 2

Non-specific:
Immediate response, same for all pathogens.
Includes physical barrier (skin) and phagocytosis.

Specific:
Slower response, specific to each pathogen.
Cell-mediated response (T lymphocytes).
Humoral response (B lymphocytes).

Question 3

What is the process of Phagocytosis?

Which defence system is it a part of?

Answer 3

Part of the non-specific defence mechanism.
Involves the engulfment and destruction of pathogens by phagocytic white blood cells.

Phagocyte extends the pseudopodia to engulf pathogen, forming a vesicle. Phagocytic vesicle is formes and fuses with lysosome. Hydrolytic enzymes within lysosome break down microbes and the indigestible material is discharged from cell.

Question 4

What are the two specific defence mechanisms of the human body?

Answer 4

Humoral response and cell-mediated response.
Humoral response involves B lymphocytes.
Cell-mediated response involves T lymphocytes.

Question 5

What are antigens?

Answer 5

Proteins or glycoproteins embedded in the cell-surface membrane of a pathogen/virus that is seen as foreign and stimulates the production of antibodies.

Antigens can be on the surface of a pathogen, on cell-surface membrane on other organisms of same species (organ transplant), abnormal body tissue (cancer) or in a toxin.

Antigens are non-cell phantogens whereas cell phantogens are seen by the body as part of itself so isn’t attacked.

Question 6

What are antibodies?

Which specific defence mechanism are they part of?

Answer 6

Proteins produced by B-lymphocytes, specifically plasma cells when exposed to a specific antigen. Part of the humoral response. Found in blood plasma, tissue fluid and breast milk

Question 7

What is Agglutination?

Answer 7

Antibody-antigen complexes cause agglutination.
Antibodies can’t destroy pathogens directly.
Antibodies instead use the two antigen binding sites to bind to two different pathogens with the same antigen on, making these pathogens clump together. This clump of pathogens and antibodies can then be engulfed and destroyed by phagocytosis.

Question 8

How is Phagocytosis stimulated by antibodies?

Answer 8

One type of antibody attaches to an antibody on surface of pathogen, identifies it for destruction by phagocytic white blood cells.
Antibody binds to receptor on phagocyte, so phagocyte can now engulf pathogen.

Question 9

What is the Humoral (clonal selection) response?

Answer 9

Different types of B cell.
• B cells secrete small amounts of specific antibody into their cell-surface membrane.
• If specific antigen binds to specific, complementary antibody on B lymphocyte, B lymphocyte is stimulated to divide by mitosis.
• Results in large number of identical plasma cells which secrete the same specific antibody into blood.
• These antibodies can form antibody-antigen complexes and results in destruction of antigen if it appears.
• Helper T Cells stimulate B cells to divide to form plasma B cells.
• This is the primary response and can take up to 72 hours.
• Memory B cells can divide and develop into plasma cells if antigen is encountered again, this results in plasma cells secreting specific antibody faster and in larger concentration (secondary response).

Question 10

What is antigenic variation?

Answer 10

Some microorganisms like influenza virus have high mutation rate = antigenic variation because different strains will have different antigens so you can be immune to one strain but if another strain has different antigens you’ll have to go through primary and secondary response again.

Question 11

What is the Cellular (cell-mediated) response?

Answer 11

Involves T cells. T cells don’t produce antibodies but have receptors on cell-surface membrane that can detect specific antigens.

• Antigen usually exposed to T cell by antigen-presenting cells (phagocytes) which stimulate T cell go divide by Mitosis.
• Can become Cytoxic T Cells, Helper T cells or Memory T cells).

Question 12

What are the two types of Passive Immunity?

Answer 12

Natural Passive: antibodies obtained via breast milk or placenta. Short-term protection, body doesn’t produce own antibodies.

Artificial Passive: pre-former specific antibodies injecter after infection has occurred (snake venom), short-term protection.

Question 13

What are the two types of Active Immunity?

Answer 13

Natural Active: exposes to pathogen/antigen but infection, body produces its own antibodies and memory cells so long term protection (usually).

Artificial Active: vaccines (exposed to dead/attenuated version of the pathogen but still stimulates production of antibodies and memory cells so usually long term protection).

Question 14

How do vaccinations work?

Answer 14

Vaccines inject a dead or attenuated (unable to spread disease) pathogen into the blood. This activates the specific immune system response, meaning that the next time the person encounters the pathogen, they have a secondary response (quicker) and are protected.

Question 15

What is Herd Immunity?

Answer 15

Higher percentage of population vaccinated, less widespread transmission and the pathogen is unlikely to encounter an unprotected individual and even if it does, it can’t spread beyond that person.

Question 16

What is HIV?

How is it transmitted?

Answer 16

Human Immunodeficiency Virus (HIV) infects people and leads to Acquired Immune Deficiency (AIDs).

It is a retrovirus (RNA as it’s nucleic acid) transmitted by infected blood, sexual fluids, sharing of needles and mother to baby (placenta, during child birth or breast milk).

Question 17

How does HIV develop into AIDs?

Answer 17

AIDs occurs when the Helper T Cells are destroyed and the immune system is ruined.

Question 18

How does HIV replicate?

Answer 18

• Virus attaches to Helper T Cell use glycoprotein spikes which are complementary to specific protein receptor sites on Helper T Cells.
• Lipid envelope fuses with cell-surface membrane, viral RNA and reverse transcriptase gets injected/released.
• Viral DNA formed in T Cell, enters nucleus and replicates with host DNA.
• Viral DNA can remain latent for up to ten years.
• Active viral DNA controls synthesis or viral proteins and viral RNA.
• HIV particles are assembled and burst out of cell, infect other Helper T Cells.
• Memory cells can also be destroyed.

Question 19

What are the symptoms of HIV?

Answer 19

Short flu-like illness after infection.
Third phase brings AIDS-related complex (ARC) and with it these symptoms: bacterial, viral and fungal infections (AIDS-defining illnesses). Loss of weight, severe diarrhoea.

Question 20

What are Monoclonal Anitbodies?

How are they made?

Answer 20

Identical antibodies with the same tertiary structure so same antigen binding site. Produced in large amounts from single clone of antibody-producing B cells (fuse a tumour cell with a B cell and stimulate it to replicate with electricity?)

Question 21

How are Monoclonal Antibodies used?

Answer 21

• Target medicine to specific cell types by attaching therapeutic drug to an antibody. Cancer cells have different antigens to healthy so toxic drug can be attached to specific monoclonal antibodies and cancer cells are killed. (Can make Cytotoxic T Cells more aware of tumour).
• Can be used in ELISA testS to detect either specific antibodies or antigens.

Question 22

What are the two types of ELISA test and what are they used for?

Answer 22

Direct and Indirect ELISA.
Used to diagnose illnesses in people.
Direct ELISA tests for antigens, can determine if a pathogen is present.
Indirect ELISA tests for antibodies, can determine if individual has antibodies against pathogen which indicates previous or current infection.

Question 23

What is the Direct ELISA Test?

Answer 23

Stage One: Plastic tray with wells are coated with monoclonal antibody specific to antigen of pathogen being detected. Monoclonal antibodies are irreversibly bound to the plastic.

Stage Two: Fluid sample added to well. If molecules of specific antigen are present they will bind to form antibody-antigen complexes. Plastic tray is washed (so that no molecules except antigens are left if present so no false positive result is given).

Stage Three: Second monoclonal antibody specific to same antigen with enzyme attached is added. Second antibody can only bind if the antigen is present. Plastic tray is washed again so only second monoclonal antibodies are left if the antigen is present so no false positive result is given.

Stage Four: Suitable colourless substrate for enzyme is added. If enzyme is still present, it will catalyse reaction and colourless substrate will be converted to coloured product. A colour change means the antigen is present otherwise the enzyme would not be present.

Question 24

What is the Indirect ELISA Test?

Answer 24

USED TO DETECT HIV.
Stage One: Plastic tray with wells are coated with antigens that are irreversibly bound to the plastic.

Stage Two: Fluid added to plastic tray. If disease was/is present in patient, antibodies specific to antigens will be present and will bind to form antibody-antigen complexes. Plastic tray is washed (so that no molecules except antibodies are left if present so no false positive result is given).

Stage Three: Second monoclonal antibody specific to the antibody with enzyme attached is added. Second antibody can only bind if the first antibody is present. Plastic tray is washed again so only second monoclonal antibodies are left if the antibody is present so no false positive result is given.

Stage Four: Suitable colourless substrate for enzyme is added. If enzyme is still present, it will catalyse reaction and colourless substrate will be converted to coloured product. A colour change means the first antibody is present otherwise the enzyme would not be present.

Question 25

What is the structure of an antibody?

Answer 25

Four polypeptide chains joined by disulphide bridges.
Two heavy chains, two light chains.
Constant region and variable regions

Constant region is light chains, sequence of amino acids that is the sam for all molecules of same type of antibody. Hinge that bends to form Y shape.

Variable religions at ends of light chains (at the top of the Y bits). Amino acid sequence here varies between different antibodies which are specific to different antigens and contains an antigen binding site between the light and heavy chains. Antigen binding site has specific tertiary structure and forms antibody-antigen complex.

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Question 26

How does a phagocyte become an antigen-presenting cell?

Answer 26

When the phagocyte digests the pathogen, it can take the antigens from its surface and embed them in its cell-surface membrane.

Question 27

Describe the function of the three types of T cell produced in the cellular response.

Answer 27

• Cytotoxic T cells use complementary receptors to attach to specific antigen and secrete chemicals which signal the cell go commit suicide via lysosomes bursting releasing hydrolytic enzymes OR by perforating cell-surface membrane, causing death.
• Helper T Cells stimulate B lymphocytes to divide into plasma cells.
• Memory T cells stay in blood after infection cleared and produce secondary response (quicker) if same antigen/pathogen is encountered.

Question 28

What is the structure of HIV?

Answer 28

Protein capsid- protects RNA.
Viral envelope- piece of plasma membrane from last human host cell.
Surface glycoproteins- attach to CD4 receptors on host cells (CD4 only found on T cells and macrophages).
Two copies of reverse transcriptase- transcribes RNA into DNA.
Two identical strands of RNA.

Question 29

What are the four phases of HIV?

Answer 29

Phase One: Body produces HIV antibodies (can be used to detect HIV), short flu-like illness can occur. (Asymptomatic phase).

Phase Two: Antibody positive phase (HIV positive phase). Between infection and onset of clinical signs. Can last weeks to years.

Phase Three: AIDS-related complex (ARC). Not life threatening but bacterial, viral and fungal infections can occur and severe diarrhoea and weight loss can be seen. Reduction in number of T Helper Cells.

Phase Four: Infections of body organs, possible secondary cancers. Weight loss. Forms of pneumonia is usually what kills AIDS patients as their immune system can’t fight infection.

Question 30

What does ELISA stand for?

Answer 30

Enzyme Linked Immunosorbent Assay.